1. Field of the Invention
An apparatus consistent with the present invention relates to an objective optical system employing a gradient index (GRIN) lens and, more particularly, to an objective optical system employing a GRIN lens that can correct aberrations and can be used in an ultra-small optical pickup.
2. Description of the Related Art
Objective lenses are the main parts used in optical disk drives (ODDs), such as compact disks (CDs) or digital versatile disks (DVDs), to record or reproduce information. Objective lenses condense laser beams emitted from laser diodes, which are light sources, and focus the laser beams on recording surfaces of optical disks to record information. Objective lenses may also condense laser beams reflected from the optical disks and send the laser beams to photo-detectors to reproduce the information recorded on the optical disks.
As demands for mobile systems (such as MP3s, personal digital assistants (PDAs), digital cameras, and camcorders) increase, studies of portable ultra-small optical disk drives are being vigorously made. Accordingly, ultra-thin optical pickups are being developed. In order to store more than 1 GB of information per optical disk, ultra-thin optical pickups use semiconductor laser beams having a blue wavelength of 400 nm to 410 nm and employ objective lenses having a numeral aperture (NA) higher than 0.85 to record and reproduce information on optical disks having a diameter smaller than 30 mm.
Objective lenses with a high NA should be able to remove chromatic aberration caused by a change in the wavelength of laser beams when temperatures of laser diodes are changed, and also correct spherical aberration and coma aberration according to the high NA.
To remove the chromatic aberration and coma aberration, three conventional methods have been used. The first method is to combine a concave lens made of a low dispersion optical glass with a convex lens made of a high dispersion optical glass. However, this method increases the weight and size of objective optical systems, and accordingly, is not suitable for ultra-thin optical pickups. The second method is to reduce chromatic aberration by using a low dispersion aspherical lens. However, the refractive index of the low dispersion optical glass used in this method is small and a curvature should be increased in order to correct coma aberration with respect to a high NA. Therefore, the second method is limited in correcting coma aberration generated in objective lenses with a high NA in ultra-small optical pickups.
The third method, as shown in FIG. 1, is to use a hybrid lens formed by combining a refractive lens R with a diffractive optical lens DOE. However, with this method, it is not easy to form a fine pitch when manufacturing the diffractive optical lens DOE and to obtain sufficient transmittance. In particular, for a doublet hybrid lens, it is difficult to bond the refractive lens R to the diffractive optical lens DOE, thereby requiring an additional process. Moreover, when the diffractive optical lens DOE is directed toward an optical disk, friction upon contact with the optical disk or contamination with particles may occur. In addition, light beams of an order other than zero and ±1 may become noise.
To solve these problems, U.S. Pat. Nos. 5,912,770, 6,233,217, 6,330,116, and 5,612,574 disclose optical systems employing gradient index (GRIN) lenses. GRIN lenses are one of the most important optical elements in micro optics. The GRIN lenses, which are used to collimate light beams emitted from light sources and condense light beams, are usually manufactured using an ion exchange technology. For example, FIG. 2 schematically illustrates an objective optical system in which a GRIN lens GR and a diffractive lens DR are used. However, a refractive index of the GRIN lens GR varies only in a radial direction and, in fact, all the GRIN lenses disclosed in the aforesaid references have refractive indices which vary only in one direction, either radial or axial. Hence, an additional optical element for correcting aberration is required and the conventional arts are not suitable for ultra-small optical pickups.